This application is based on application Nos. 2000-229400, 2000-259672, 2000-259673, 2000-259674, 2000-264524, and 2000-264525 filed in Japan, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a duplexer, a weak signal extracting filter or the like, and more particularly to a surface acoustic wave device in which a plurality of SAW filter elements are housed in a single container.
2. Description of Related Art
Recently, a surface acoustic wave filter formed by surface acoustic wave elements (hereinafter referred to as SAW filter), is used in a variety of communication fields. Because of its characteristics of high-performance, compact design, mass-productivity and the like, this SAW filter plays an important role of the diffusion of small-size communication devices such as cellular phones or the like.
The requirements for miniaturization and higher performance of small-size communication devices are increasing constantly in severity. Recently commercialized is a product in which a single container houses a plurality of SAW filters (e.g., a front-end SAW filter and an inter-stage SAW filter).
Examples of the product in which a single container houses a plurality of SAW filters, include a duplexer in which a signal in the reception frequency band is separated from a signal in the transmission frequency band, and a dual-or triple-band weak signal extracting filter in which signals in a plurality of frequency bands are separated from one another.
For example, a duplexer has, in a single container, a SAW filter element serving as a reception filter, and a SAW filter element serving as a transmission filter. A weak signal extracting filter comprises a front-end SAW filter element, an amplifying circuit and an inter-stage SAW filter element, and the front-end and inter-stage SAW filter elements are disposed in a cavity of the same container.
In any case, it is important to restrain floating capacitance among a plurality of SAW filter elements to prevent crosstalk from occurring. Particularly, in the weak signal extracting filter, the front-end SAW filter element and the inter-stage SAW filter element have bands of which center frequencies are approximately identical with each other, and these two SAW filter elements are required to be driven at the same time. Accordingly, it becomes important to achieve isolation (reduce crosstalk) between these two SAW filter elements as much as possible.
More specifically, an increase in crosstalk introduces the following problem. When the front-end SAW filter element and the inter-stage SAW filter element are connected in cascade to each other by external wirings, there cannot often be obtained the total attenuation amount which can theoretically be expected from the individual attenuation amounts of the two SAW filter elements. This is because the total attenuation amount of the two SAW filter elements is restricted by the crosstalk between the front-end SAW filter element and the inter-stage SAW filter element. Actually, such a device is often used with an amplifying circuit interposed between the two SAW filter elements. In such a case, too, a similar problem arises.
A method of achieving isolation between two SAW filter elements as above-mentioned is for example disclosed by Japanese Patent Laid-Open Publication No. 11-205077.
According to the technique in the Publication above-mentioned, a surface acoustic wave device is arranged such that two SAW filter elements are housed in a cavity of a container, and that the input pads, output pads, and ground pads of the container are respectively electrically connected to the input electrodes, output electrodes and ground electrodes of the SAW filter elements. It is particularly disclosed that a plurality of ground pads are disposed in the cavity of the container and that each ground electrode of one SAW filter element and each ground electrode of the other SAW filter element are respectively connected to different ground pads.
The prior art above-mentioned prescribes only the connection relationship between the SAW filter elements and the ground pads formed in the cavity.
The inventors of this application had conducted a various tests and found that there occurred crosstalk resulting from the connection means between the ground electrodes of the SAW filter elements and the ground pads of the container. More specifically, the inventors found that crosstalk occurred as resulting from not only the bonding wires, but also the shapes of ground conductor films formed on the SAW filter mounting surface of the cavity in which the SAW filter elements are mounted, and the arrangement of the ground external terminals.
(A) Ground Conductor Film
A surface acoustic wave device of prior art is shown in FIGS. 16 and 17. FIG. 16 is a perspective view of the surface acoustic wave device with its metallic lid omitted, and FIG. 17 is a perspective view thereof with SAW filter elements omitted.
Shown in FIGS. 16 and 17 are a front-end SAW filter element 100, an inter-stage SAW filter element 200, a concaved container 300, a ground pad 136 formed on a step portion of the container 300 and connected to a ground electrode 101 of the front-end SAW filter element 100 through a bonding wire, a ground pad 139 formed on a step portion of the container 300 and connected to a ground electrode 102 of the inter-stage SAW filter element 200 through a bonding wire, a ground conductor film 500 formed substantially on the entire surface of the mounting surface of a cavity 317 of the container 300, and ground external terminals 145, 146, 147, 148 formed on the external lateral sides of the container 300 and connected to a metallic lid (not shown).
As shown in FIG. 17, the ground conductor film 500 is connected to the ground pad 136 and electrically connected to the ground external terminal 145 formed on the external lateral side of the container 300. Likewise, the ground conductor film 500 is connected to the ground pad 139 and electrically connected to the ground external terminal 146. The ground conductor film 500 is connected to a ground pad 138 and electrically connected to the ground external terminal 147, and the ground conductor film 500 is connected to a ground pad 137 and electrically connected to the ground external terminal 148. The ground external terminals 145, 146, 147, 148 are respectively disposed at a plurality positions. This is for assuring the ground potential of the ground conductor film 500.
According to the arrangement above-mentioned, for example when the ground pad 136 connected to the front-end SAW filter element 100 and the ground pad 139 connected to the inter-stage SAW filter element 200, are commonly connected to the ground conductor film 500, the ground electrodes of the two SAW filter elements 100, 200 are consequently commonly connected to the ground conductor film 500 in the container 300.
When a surface acoustic wave device having this ground conductor film 500 is mounted on a circuit board (not shown), there are disadvantageously generated, between the ground potential of the ground conductor film 500 and the ground potential of the circuit board, parasitic impedances resulting from the ground external terminals 145, 146, 147, 148 and parasitic impedances resulting from the ground conductor film 500. These parasitic impedances are generated, between the ground potential of the ground conductor film 500 and the ground potential of the circuit board, through the ground conductor film 500 commonly electrically connected to the SAW filter elements 100, 200. Accordingly, the parasitic impedances generated at the side of one SAW filter element 100 also exert influence on the other SAW filter element 200.
Accordingly, the connection of the ground external terminals formed on the external sides of the container to the ground conductor film, causes the ground levels of the two SAW filter elements to be identical with each other. Therefore, even though each of the two SAW filter elements is for example connected to each peculiar ground pad as shown in Japanese Laid-Open Patent Publication No. 11-205077, the crosstalk between the SAW filter elements cannot sufficiently be restrained and stable characteristics cannot therefore be obtained.
As to why stable characteristics cannot be obtained, the following would be considered. The common ground conductor film 500 is formed in a wide area of the mounting surface of the cavity. In other words, the area of the ground conductor film 500 is basically broad. This makes it difficult to maintain the entire ground conductor film at the ground potential. Further, the broad area increases the degree to which parasitic impedances are generated.
On the other hand, Japanese Laid-Open Patent Publications Nos. 10-224175 and 10-209800 disclose a surface acoustic wave device in which the ground conductor films formed on the mounting surface of the cavity of the container, are electrically separated from one another.
However, there are disclosed (ii) a surface acoustic wave device in which the ground conductor film connected to the input side of a single SAW filter element, is separated from the ground conductor film connected to the output side of the same SAW filter element (Japanese Laid-Open Patent Publication No. 10-224175), and (ii) a surface acoustic wave device in which there are electrically separated, from each other, the ground conductor films connected to the reception and transmission sides of a comb-like electrode of a single SAW filter element (Japanese Laid-Open Patent Publication No. 10-209800). Any of the techniques above-mentioned is arranged to improve the attenuation characteristics of a signal outside of the band in a single SAW filter element. Accordingly, no prior art has been disclosed as to the technology of separating the ground conductor films from one another to prevent crosstalk among a plurality of SAW filter elements used in a single container as done in a duplexer or a weak signal extracting filter container.
It is an object of the present invention to provide a surface acoustic wave device in which the ground connection structures between the ground electrodes of SAW filter elements and the ground external terminals on the external lateral sides of a container, are independently formed, inside of the container, for the respective SAW filter elements, thereby to effectively restrain the mutual influence of parasitic impedances resulting from the ground connection structures of the SAW filter elements, resulting in stable filter characteristics.
(B) Arrangement of Ground External Terminals
In a surface acoustic wave device, the arrangement of the input and output external terminals has been decided with priority given to the convenience of a user who mounts the surface acoustic wave device on a circuit board. As a result, in a surface acoustic wave device having, in a single container, front-end and inter-stage SAW filter elements as connected in cascade to each other, an input external terminal 41 of a front-end SAW filter element 100 and an output external terminal 44 of an inter-stage SAW filter element 200, are disposed at the same lateral side of a container 300, as shown in FIG. 19.
When the input/output external terminals are disposed as shown in FIG. 19, this involves the likelihood that the cascade-connection characteristics expected from the respective filters are not sufficiently obtained even though provision as shown in Japanese Laid-Open Patent Publication No. 11-205077 is made. More specifically, the attenuation amount of electric characteristics at the time when the front-end and inter-stage SAW filter elements are connected in cascade to each other, is disadvantageously governed by crosstalk or isolation characteristics between the input external terminal of the front-end SAW filter element and the output external terminal of the inter-stage SAW filter element.
More specifically, the arrangement of the input/output external terminals as shown in FIG. 19, not only shortens the distance between the input external terminal 41 of the front-end SAW filter element 100 and the output external terminal 44 of the inter-stage SAW filter element 200, but also increases, in view of distance and arrangement, both the interference among the pads of the bonding wires inside of the container 300, and the crosstalk among the bonding wires electrically connecting the input/output electrodes of the SAW filter elements 100, 200 to the pads.
It is an object of the present invention to provide a surface acoustic wave device capable of effectively restraining the influence of crosstalk among the SAW filter elements thereby to achieve good cascade-connection attenuation characteristics, by specifying the arrangement of the input/output external terminals of the container, i.e., the arrangement of the input external terminal for the forefront SAW filter element and the output external terminal for the last-stage SAW filter element.
The present invention provides a surface acoustic wave device in which a plurality of SAW filter elements are housed in a single container, and is characterized in that ground conductor films are respectively disposed in the areas where the SAW filter elements are housed in a cavity portion, and that all the ground conductor films respectively formed in the areas where the SAW filter elements are respectively housed, are electrically separated from one another within the container.
The present invention provides a surface acoustic wave device in which a plurality of SAW filter elements are housed in a single container, and is characterized in that the container is rectangular, that a plurality of SAW filter elements are connected in cascade to one another, and that the input external terminal electrically connected to the forefront SAW filter element out of the cascade-connected SAW filter elements, is formed at or in the vicinity of a first corner portion of the outer peripheral surface of the container, and the output external terminal electrically connected to the last-stage SAW filter element is formed at or in the vicinity of a second corner portion which is located in a diagonal direction with respect to the first corner portion.
The present invention provides a surface acoustic wave device capable of considerably reducing crosstalk between two SAW filter elements.
The following description will discuss specific arrangements of the present invention with reference to the attached drawings.